The invention is a digitizer circuit in a peripheral system for a computer comprising a moveable instrument and a planar grid in which an excitation signal in one of said instrument and grid generates an induced signal in the other one of said instrument and grid, and wherein the phase difference between said excitation and induced signals is indicative of the position of said instrument with respect to said grid, comprising means for summing said excitation and induced signals to produce a sum signal and means for producing an output signal whose frequency is a function of the amplitude of said sum signal.
A hand-moveable peripheral device or instrument for a computer including a stationary planar surface under the instrument transmits to the computer digital signals indicative of the instantaneous position of the moveable instrument with respect to an X-Y coordinate system of the planar surface. One example is a mouse which is moveable over a digitizing pad. The mouse has a coil or similar device while the digitizing pad has a conductive grid. In some systems, a constant excitation signal is transmitted by the moveable instrument and the computer senses the induced response in various individual conductors in the planar surface to determine the instantaneous position of the instrument. In other systems, a variable excitation signal is transmitted from the planar surface, and the computer senses the timing of an induced response in the moveable instrument.
There are various techniques for determining the instantaneous instrument position. One is to sense current ratios across the stationary surface, as in U.S. Pat. No. 4,680,429 to Murdock et al. Another technique is to sense voltage ratios across the stationary surface, as in U.S. Pat. No. 4,603,231 to Reiffel et al. Yet another technique is to sense the timing of the response to determine the instrument position, as in U.S. Pat. No. 3,904,821 to Whetstone et al. Still another technique is to vary the frequency of an excitation signal applied to individual conductors in the grid of the stationary surface in a phase lock loop which includes the sensor coil in the moveable instrument, as in U.S. Pat. No.'s 3,983,322 and 4,022,971, both to Rodgers.
The technique to which the present invention is directed is one in which the phase of the induced signal is compared with the phase of the excitation signal to determine the instantaneous position of the moveable instrument. For example, the excitation signal may be a constant frequency RF signal transmitted by the moveable instrument, which induces RF signals of different phases in different ones of the individual conductors in an X-Y conductor grid in the stationary surface. A digitizer circuit generates a digital clock signal whose frequency is proportional to the phase difference between the excitation signal and the induced signal on a selected one of the conductors. An X-Y multiplexer permits the digitizer circuit to successively process the induced signal in each one of the conductors in the grid.
Whichever one of the conductors has a phase difference at the zero-phase crossover point is the one underlying the moveable instrument. Such a determination by a microprocessor, for example, is made for all conductors lying in the X-direction of the grid as well as all conductors lying in the Y-direction, thus providing the instantaneous X-Y coordinate of the moveable instrument. This technique is disclosed in U.S. Pat. No. 4,210,775 to Rodgers et al., U.S. Pat. No. 4,455,451 to Kriz and U.S. Pat. No. 4,734,546 to Landmeier. The present invention is directed to improving the digitization circuit.
A digitization circuit which is typical of the type employed in the above-referenced patents to Rodgers et al., Kriz and Landmeier is illustrated in FIG. 1. The X-Y multiplexer circuit (not shown) couples the induced signal V(z) from a selected one of the digitizing pad grid conductors (also not shown) to the input of a first amplifying stage 10. The output of the first amplifying stage 10 is filtered in a filter capacitor stage 12 and amplified in a second amplifying stage 14. A synchronous demodulator (or "analog switch") 16 alternately applies the output of the second amplifying stage 14 to one of two output conductors 18, 20 in synchronism with the excitation signal V(z)' of the moveable instrument or mouse. The conductors 18, 20 are coupled through respective resistor networks 22, 24 to respective differential inputs of an operational amplifier 26. The amplifier 26 produces an output voltage proportional to the phase difference between the excitation signal V(z)' and the induced signal V(z).
A voltage to frequency converter 28 produces a digital clock signal whose clock frequency is proportional to the output voltage of the operational amplifier 26. The voltage to frequency converter 28 has an integrator 30 comprising an amplifier 32, an input resistor 33 and a feedback integrating capacitor 34. The output of the integrator 30 is applied through a diode 36 to the A input of a one-shot multivibrator integrated circuit 38. The Q output of the multivibrator circuit 38 is connected to the integrating capacitor 34, while the Q' output provides the digital clock signal.
The voltage across the integrating capacitor 34 increases at a rate proportional to the phase difference between V(z) and V(z)'. Each time the integrating capacitor voltage ramps up to the threshold of the A input of the multivibrator circuit 38, the multivibrator 38 changes state so that the Q output resets the voltage on the integrating capacitor 34, while the Q' output transmits a clock pulse.
One problem with the digitization circuit of FIG. 1 is that the synchronous demodulator 16 is implemented in a separate integrated circuit, such as the 74HC4053 integrated circuit used in the above-referenced patent to Landmeier, for example, and is therefore a relatively expensive item. The four amplifiers, including the two in the first and second amplifier stages 10, 14, the operational amplifier 26 and the integrating amplifier 32 are implemented together in a National Semiconductor M837 integrated circuit, while the multivibrator circuit 38 is a 74HC4538 integrated circuit. Thus, there are three key integrated circuits in the digitizer circuit of FIG. 1, a relatively expensive configuration.
If it were somehow possible to eliminate the synchronous demodulator 16, one of the three integrated circuits would be eliminated, providing a significant reduction in parts count and increase in simplicity. Furthermore, if it were somehow possible to substitute a simpler device in place of the multivibrator integrated circuit 38, a further cost reduction and simplification would be achieved. However, such improvements have not seemed possible or practical in the prior art.
Accordingly, it is an object of the invention to provide a simpler digitizer circuit for use with a moveable instrument such as a mouse and a stationary surface such as a digitizer pad.
It is a further object of the invention to provide a digitizer circuit for use with a moveable instrument and digitizer pad which generates a clock signal whose frequency is proportional to the phase difference between the excitation and induced signals in the instrument/pad system without requiring a synchronous demodulator or analog switch or the like.
It is another object of the invention to provide a digitizer circuit for use with a moveable instrument and digitizer pad which generates a clock signal whose frequency is proportional to the phase difference between the excitation and induced signals in the instrument/pad system without requiring a synchronous demodulator or analog switch or the like and which employs a synchronous voltage to frequency converter.
It is a yet further object of the invention to provide a digitizer circuit for use with a moveable instrument and digitizer pad which generates a clock signal whose frequency is proportional to the phase difference between the excitation and induced signals in the instrument/pad system without requiring a synchronous demodulator or analog switch or the like and which employs a synchronous voltage to frequency converter requiring no multivibrator integrated circuit.
These and other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawings which accompany it.